Solid-state nickel-metal hydride (NiMH) type batteries may comprise a nanoporous composite layer of silica and titania as a negative electrode (anode), wherein the titania functions as the active electrode material. The composite electrode layer comprises TiOx nanoparticles that are embedded or dispersed in a porous silica (SiOx) structure. The operation principle of such a battery relies on the presence of an adsorbed water layer on the pore walls or pore surfaces of the nanoporous silica structure. The water layer provides both (a) water as a reagent in the charging and discharging reactions at the anode and cathode and (b) the means for ionic conduction (the electrolyte). As a positive electrode (cathode), a NiO layer has been proposed. The NiO layer may for example be provided by Physical Vapor Deposition. The positive electrode and the negative electrode may be separated by a thin layer of nanoporous dielectric material such as silica which also contains adsorbed water on its pore walls and acts as a solid electrolyte.
However, using NiO as a positive electrode material in such a device structure may have some disadvantages. For example, when using NiO there is a net consumption of water in the charging reaction (i.e. during battery charging). This may dry out the adsorbed surface water layer that is used for battery operation. Further, the reaction of water or hydroxyl ions with the NiO electrode is limited to a relatively thin surface layer of the NiO electrode, because water and hydroxyl ions cannot easily penetrate into the bulk of the NiO layer. In order to increase the electrode reaction rate (current) an electrode with a higher effective surface area than a planar electrode may be used. As such, a porous NiO structure may be more efficient as it allows better access of water.
As an alternative to “dry” NiO, “wet” Ni(OH)2 may be used as a positive electrode material in conventional wet nickel-metal hydride type batteries with an alkaline electrolyte. In some embodiments, Ni(OH)2 may be used in a water-based solid-state nickel-metal hydride type battery as described above, because when using such electrode there is no net consumption of water during charging/discharging and it is an intrinsically hydrated material allowing also access of water to the bulk of the material. In conventional alkaline-based NiMH and alkaline Zn batteries, Ni(OH)2 is typically applied from powders. However, due to the relatively large particle size of the powders (for example in the order of 10 micrometer) such process is not suitable for forming an electrode layer of a thin-film battery, wherein the electrode layer typically has a thickness in the order of 1 micrometer. Apart from particle-based processes, electrodeposition of Ni(OH)2 may be used for the formation of nanoporous Ni(OH)2 layers, but these electrodeposition processes are not suitable for deposition of Ni(OH)2 layers as an electrode layer of a battery type as described above, having an operation principle relying on the presence of an adsorbed water layer on the pore walls or pore surfaces of a nanoporous silica structure.